Hadrons in the Nuclear Medium Rolf Ent Jefferson Lab

Operated by the Southeastern Universities Research Association for the U.S. Department Of Energy

Thomas Jefferson National Accelerator Facility

Hadrons in the Nuclear MediumRolf Ent

Jefferson Lab

Science & Technology ReviewJune 2004

•The Baryon-Meson versus Quark-Gluon Description•Nuclear Effects in Structure Functions•Tagged Structure Functions•Using the Nucleus as a Laboratory•Summary



Campaigns and Performance Measures

The Structure of the Nuclear Building Blocks 3) How does the NN Force arise from the underlying quark and gluon structure

of hadronic matter? The Structure of Nuclei4) What is the structure of nuclear matter? 5) At what distance and energy scale does the underlying quark and gluon

structure of nuclear matter become evident?DOE Performance Measure:… and compare free proton and bound proton properties via measurement of

polarization transfer in the 4He(e,e’p)3H reaction.

QCD Lagrangian: quarks and gluonsNuclear Physics Model is an effective (but highly

successful!)model using free nucleons and mesons as degrees of

freedom.Are these, under every circumstance, the best effective degrees of freedom to chose?



JLab Data Reveal Deuteron’s Size and Shape

The nucleon-based description works down to < 0.5 fm

Combined Data ->Deuteron’s Intrinsic Shape



Is there a Limit for Meson-Baryon Models?Not really but …

… there might be a more economical QCD description.



Transition to the Quark-Gluon Description

d/dt f(cm)/sn-2

where n = nA + nB + nC + nD

s = (pA+pB)2, t=(pA-pC)2

d pn n=13 d/dt s-

11

ConventionalNuclear Theory

pA

pB

pC

pD

Scaling behavior (d/dt s-11) for PT > 1.2 GeV/c (see )

quark-gluon description sets in at scales below ~0.1 fm



Exploring the Transition Region

Now nearly complete angular distributions of D(,p)n up to E = 3 GeV with CLAS

Excellent fit of data with d/dt s-11 if starting fit at pT ~ 1.0-1.3 GeV/c.



Hadrons in the Nuclear Medium

• Nucleons and Mesons are not the fundamental entities of the underlying theory.• At high densities a phase transition must occur to a quark-gluon plasma.• At nuclear matter densities of 0.17 nucleons/fm3, nucleon wave functions overlap considerably.

• EMC effect: Change in the inclusive deep-inelastic structure function of a nucleus relative to that of a free nucleon.



Quarks in Nuclear PhysicsQuark-Meson Coupling (QMC) Model (Thomas et al.): quark’s response to mean scalar and vector field (similar to quantum hadrodynamics for nucleon’s response) quark substructure of nucleon irrelevant for bulk properties of nucleus, but is relevant for form factor and structure functions

F 2A /F

2D

(GE/G

M) Q

MC/(

G E/G

M) fre

e

Q2 (GeV/c)2 x

Effects are Precursor of Quark Gluon Plasma

0



The EMC EffectEuropean Muon Collaboration: muon scattering to measure nuclear structure functions (1982)

2F2A /F

2D

• Depletion of the nuclear structure function F2A(x) in the valence-quark

regime 0.3 < x < 0.8• Specific cause for depletion remains unclear: conventional nuclear effects or nucleon swelling?• J. Smith & G.A. Miller (2003): chiral quark soliton model of the nucleon claim: Conventional nuclear physics does not explain full EMC effect

?



EMC Effect in 3He and 4HeThe Nuclear EMC effect shows that quarks behave differently in nuclear systems.It has been extensively measured in A>8 nuclei.



EMC Effect in 3He and 4HeThe Nuclear EMC effect shows that quarks behave differently in nuclear systems.It has been extensively measured in A>8 nuclei, but current data do not differentiatebetween models with either an A-dependence or a -dependence.

Measure the shape in very light nuclei to distinguish Test models of the EMC effect in exact few-body calculations

SLAC fit to heavy nuclei(scaled to 3He)

Calculations by Pandharipande and Benhar for 3He and 4He



EMC Effect in 3He and 4HeThe Nuclear EMC effect shows that quarks behave differently in nuclear systems.It has been extensively measured in A>8 nuclei, but current data do not differentiatebetween models with either an A-dependence or a -dependence.

Measure the shape in very light nuclei to distinguish Test models of the EMC effect in exact few-body calculations

SLAC fit to heavy nuclei(scaled to 3He)

Calculations by Pandharipande and Benhar for 3He and 4He

Projected uncertainties for an experiment to run this year (November)



EMC Effect in R = L/T

EMC Effect is measured as ratio of F2A/F2

D

In Bjorken Limit: F2 = 2xF1 (transverse only)There should be medium effects also in FL, or in R = L/T !

E. Garutti, Ph.D UvA 2003,

HERMES: RA = RD within 25% JLab: Rp @ low Q2, RD + RA soon

Should be able to see effects, if any.



R in Nuclei – Link with Neutrino Community

Mass difference between neutrino flavors: M2 ~ E/L, with E the energy and L the distance source-detector L was fixed when M2 was thought to be larger

energy of interest ~ few GeV ~ JLab energy!

physics derived from Monte Carlo models folding poorly known beam properties with the best known response of a detector to the creation of various particles in interactions All conventional Nuclear Physics needs to be known for few-GeV probes and included, both for the axial and vector couplingsApproach I : High Luminosity few-GeV Experiments

Approach II : Electron scattering experiments to constrain Monte CarloApproach III: Do both to extract axial information from difference

Neutrino community (Bodek et al.) involved in:I) Approved experiment to measure R = L/T in Nuclei in Hall CII) Analysis of existing data on low-energy

transparencies and multi-pion production in Hall B



A nuclear medium has an average density, ρo, of 0.17 GeV/fm3.

A typical distance for 2 nucleons participating in a short-range correlation (SRC) is ~1.0 fm the local density can increase by a factor of ~4: this is comparable to the density of neutron stars.

Nucleons participating in a SRC are deeply bound, i.e. their structure should be modified, like their shape or quark distributions.

x = Q2/2M > 1 can be used to select quarks inside nucleon participating in a SRC superfast quarks! Nucleus

NeutronProton

o = 0.17 GeV/fermi3

4o

EMC Effect at large x (> 1)



EMC Effect at large x (> 1)E89-008 @ 4 GeV:Iron cross sections vs (,Q2)Structure Functions vs

x (as Q2 ) “scaling” regime sets in at Q2 > 3

X = 1Q2 = 3} = 0.80

F 2Fe

/A

F 2Fe

/A



EMC Effect at large x (> 1) – cont.

SLAC NE3E89-008 (1996)E02-019(experiment to start data taking June 26th)

Q2 > 3!

Note: x = 3, Q2 = 5 nucleon with 1.78+ GeV/c!(similar for x = 1.5, Q2 = 10)

Acceptance plot of (x,Q2) of SLAC and JLab experiments

x



F2Neff (x,Q2 , )

F2Nfree (x,Q2 , )

Modification of the off-shell scattering amplitude (Thomas, Melnitchouk)

Suppression of “point-like configurations”

Color delocalization

6-quark bags(C. Carlson et al.)

P = 320 MeV/cM* ~ 823 MeV

Tagged Structure FunctionsGoal: Tag nucleon (here, neutron) participating in SRC directly D(e,e’ps)n

Already measured in CLAS for 300 < P < 600 MeV/c (proton angles > 107o)



Preliminary Results: x* (= Q2/2pnq)

dependence

Q2 = 1.8 GeV2

Proton angles > 107.5O

Several different proton momenta

Vertical axis: structure function F2n(x*,Q2)

x*

300 MeV/c

M* = 0.84 GeV

340 MeV/c

M* = 0.81 GeV

390 MeV/c

M* = 0.77 GeV

460 MeV/c

M* = 0.70 GeV

560 MeV/c

M* = 0.54

0.2 0.7x*0.2 0.7x*0.2 0.7

F 2n(x

* ,Q2 )



Hadrons in the Nuclear Medium

• Nucleons and Mesons are not the fundamental entities of the underlying theory.• At high densities a phase transition must occur to a quark-gluon plasma.• At nuclear matter densities of 0.17 nucleons/fm3, nucleon wave functions overlap considerably.

• EMC effect: Change in the inclusive deep-inelastic structure function of a nucleus relative to that of a free nucleon.



• Free electron-nucleon scattering

(see also talk by Kees de Jager)

• Bound nucleons evaluation within model

• Reaction mechanism effects minimal for A(e,e’p) about Pm = 0

Form Factors in the Nucleus

Polarization transfer in 4He(e,e’p)3H

• E93-049 (Hall A): Measured 4He(e,e’p)3H in quasi-elastic kinematicsfor Q2 = 0.5, 1.0, 1.6 and 2.6 (GeV/c)2 using Focal Plane Polarimeter

• Extracted “Superratio”: (P’x/P’z) in 4He/(P’x/P’z) in 1H

RPWIA RDWIA GE P’x (Ei + Ef) e

GM P’z 2m 2__ __ _____ __= - tan

GE P’x (Ei + Ef) e

GM P’z 2m 2__ __ _____ __= - tan

~~





• Extracted “Superratio”: (P’x/P’z) in 4He/(P’x/P’z) in 1H

• Compared to calculations by Udias without and with inclusion of medium effects predicted by Thomas et al. (Quark Meson Coupling model).

Medium Modifications of Nucleon Form Factor

RPWIA RDWIA +QMC





• Extracted “Double Superratio”: [(P’x/P’z) in 4He/(P’x/P’z) in 1H]/[PWIA ratio]

• After corrections for FSI effects: deviation from 1 a measure of the medium dependence of GE

p/GMp







• Extracted “Double Superratio”: [(P’x/P’z) in 4He/(P’x/P’z) in 1H]/[PWIA ratio]

• After corrections for FSI effects: deviation from 1 a measure of the medium dependence of GE

p/GMp


•New proposal approved by PAC24




Hadrons in the Nuclear MediumRolf Ent

Jefferson Lab

Science & Technology ReviewJune 2004

•The Baryon-Meson versus Quark-Gluon Description•Nuclear Effects in Structure Functions•Tagged Structure Functions•Using the Nucleus as a Laboratory

Any signatures for the onset of non-hadronicdegrees of freedom and QCD dynamics?



Nuclear Transparency

Ratio of cross-sections for exclusive processes from nuclei to nucleons is termed as Transparency

= free (nucleon) cross-sectionparameterized as =

Experimentally = 0.72 – 0.78, for p

Nucleons NucleiA + B’ C + D + XA + B C + D

N

Exclusive Processes

(A)

o

T = o

(A)



Total Hadron-Nucleus Cross Sections

Hadron– Nucleustotal cross sectionFit to

p p

_

Hadron momentum60, 200, 250 GeV/c

< 1 interpreted as due to the strongly interacting nature of the probe A. S. Carroll et al. Phys. Lett 80B 319

(1979)

= 0.72 – 0.78, for p, k



Nuclear Transparency

Traditional nuclear physics calculations (Glauber calculations) predict transparency to be nearly energy independent .

T

1.0

5.0Energy (GeV)

Ingredients• h-N cross-section

• Glauber multiple scattering approximation

• Correlations & FSI effects.

hN

For light nuclei very precise calculations of T are possible.



Search for Color Transparency in Quasi-free A(e,e’p) Scattering

From fundamental considerations (quantum mechanics, relativity, nature of the strong interaction) it is predicted (Brodsky, Mueller) that fast protons scattered from the nucleus will have decreased final state interactions

Hall C – E94-139

Constant value line fits givegood description 2/df = 1

Conventional Nuclear Physics Calculation by Pandharipande et al. (dashed) also gives good description

No sign of CT yet



A(e,e’p) Results -- A Dependence

Fit to o

= constant = 0.75 for Q > 2 (GeV/c) 2 2

Close to proton-nucleus total cross section data!



qqq vs qq systems

There is no unambiguous, model independent, evidence for CT in qqq systems.

Small size is more probable in 2 quark system such as pions than in protons.

Onset of CT expected at lower Q2 in qq system.

Formation length is ~ 10 fm at moderate Q2 in qq system ( larger than nuclear radius)

Onset of CT related to onset of factorization required for access to GPDs in deep exclusive qq production

(B. Blattel et al., PRL 70, 896 (1993)



A(,dijet) Data from FNAL

Coherent diffractive dissociationwith 500 GeV/c pions on Pt and C.

+

Fit to A0

> 0.76 from pion-nucleus total cross-section.

Aitala et al., PRL 86 4773 (2001)

Claim: Full CT effect observed by Q2 ~ 10 (GeV/c)2

A(e,e’) and A(e,e’) experiments at JLab pushing Q2 rangeA(,-p) pushing t range



Results from E94-104 ( n -> - p in 4He)

• Calculations use Glauber theory and correlations from Argonne v14 and Urbana VIII• CT estimated from quantum diffusion model, normalization can be chosen arbitrarily• Data show t-dependence seemingly at variance with traditional nuclear physics• Clear need for extension to higher t-values



A Pion Transparency Experiment

JLab Experiment E01-107: A(e,e’) on H, D, C, Cu, Au

Measurable effect predictedfor Q2 < 5 (GeV/c)2

(Miller et al., Ralston et al.)

Projected uncertainties for experiment to start this July (E = 5.75 GeV)



CT and Quark Propagation through Nuclei

Search for Color Transparency

Measure absorption vs. Q2 at fixed coherence length lc

Compare absorption in deuterium, carbon, aluminum, and iron

Quark Propagation through Nuclei

Measure attenuation and transverse momentum broadening of hadrons (p, K) in DIS kinematics

Compare absorption in deuterium, carbon, iron, tin, and lead

E02-110 E02-104EG2



CLAS EG2 - Online Results (2003)

iron

lead

deuterium

K0 short, D+Fe K0 short, D+Pb

(Simple vertex cut)

(Simple vertex cut)

27 M D+Pb electrons (5% of total)

Q2

W



SummaryIn principle both baryon-meson and quark-gluon descriptions of the nucleus should work, question is rather what is the most efficient description of observables?JLab is pushing both descriptions to the limit:1) A description of the deuteron elastic scattering data in terms of conventional

nuclear physics works to much smaller distance scales (0.5 fm) than expected.2) Deuteron photodisintegration experiments reveal the underlying quark-gluon

degrees of freedom at a distance scale smaller than 0.1 fm.3) First indication that a nucleus is not merely a set of nucleons, the nuclear EMC

effect, is under scrutiny for cases where nucleons are strongly bound (x > 1, tagged structure functions). Can alternatively view this as scattering from superfast quarks.

4) A second indication is given by the observed medium dependence of proton form factors, considered a precursor of the onset of a quark gluon plasma.

5) Proton transparency data can be well described by conventional nuclear physics;6) But have seen hints of QCD dynamics (Color Transparency) in meson production

data. Full effects of CT may be anticipated at Q2 < 10 here, presently under investigation.Data show beautiful balance between conventional nuclear

physics descriptions and QCD effects.This topic: some experiments complete, many ran/to run in FY’03 and ‘04

Documents

Hadrons in the Nuclear Medium Rolf Ent Jefferson Lab